Position sensors for sensing angular displacement of rotating automotive parts, such as toothed wheels and shafts are generally known. Angular displacement is generally recognized as a useful control and diagnostics parameter. Position sensors may be of the conventional hall effect or variable reluctance type, positioned adjacent a rotating automotive part having teeth or notches about its circumference. As the part rotates, the passage of the teeth or notches disturbs the field to which the sensor is exposed. The sensor transduces the field disturbances and outputs a signal interpreted by control or diagnostics hardware as incremental or relative displacement events. Such sensors are typically expensive and are sensitive to field and electrical disturbances.
For automotive applications requiring absolute angular position information, such incremental angular displacement information is not enough. Synchronization information is needed to provide an absolute angular position context within which the incremental displacement information may be interpreted. For example, an absolute position input may be provided when the automotive part is at a known rotational position. A counter of displacement events may be initialized to a value corresponding to the known rotational position upon receiving the absolute position input. The counter may then be updated when displacement events occur and may then be assumed to indicate absolute angular position. Counter initialization may occur periodically, such as following system start-up or following a loss of memory, and counter value verification may occur whenever the absolute position input is received. The approach described in co-pending U.S. patent application Ser. No. 08/249,409, filed May 26, 1994, now U.S. Pat. No. 5,491,632 assigned to the assignee of this application, provides both relative and absolute angular position information. A periodic signal is generated and interpreted through timing circuitry relying on differentiation operations. The timing circuitry is complex and costly, and is sensitive to pulse width variations in the periodic signal, such as may be caused by rapid angular rotation of the measured part. The differentiation operations are edge-sensitive and thus are noise sensitive. The accuracy of this costly and complex prior approach may be reduced due to the above-described sensitivities, especially in applications associated with rapid angular rotation of the measured part, or with significant noise, such as many automotive applications.